The present invention pertains to a method for the operation of a high-power electron beam employed for the vaporization of materials.
Metals and metal alloys of high quality can be produced by means of an electron beam melting process. The use of an electron beam as a heat source for melting metals and alloys has the advantage that very complex melting processes can be implemented, because the electron beam is deflectable and thus can reach different places on the surface of a metal block or a metal melt.
Nearly any material can be effectively vaporized with the aid of electron beam technology. The vaporization rate is roughly 100 times greater than that of the sputtering process. Apart from the standard processes with aluminum, materials with a high melting point and high vaporization temperature are of particular interest for the electron beam vaporization technique. Among these materials are, for instance, Cr, Co, Ni, Ta, W, alloys thereof or oxides like SiO.sub.2, Al.sub.2 O.sub.3, ZrO.sub.2, MgO. Electron beam technology also provides the required stable and uniform vaporization rates for reactive vaporization, such as Al+O.sub.2.fwdarw.Al.sub.2 O.sub.3.
A particularly important field of application of electron beam vaporization is represented by the coating of large surfaces with various materials, for instance the coating of magnetic tapes with CoNi alloys or the coating of films for the packaging of foodstuffs (See DE-OS 42 03 632 and the counterpart U.S. Pat. No. 5,302,208).
An additional field of application is the corrosion-preventive coating of turbine blades, where, for instance, a layer 100 to 200 .mu.m thick of MCrAlY is applied and an additional heat-attenuating layer of 100 to 200 .mu.m of yttrium or stabilized ZrO.sub.2 is added, so that the service of the turbine vanes is increased.
The main advantage of electron-beam coating lies in the high power density in the focal point of the electron beam, which may amount to as much as 1 MW/cm.sup.2. Due to this high power density, a high surface temperature results, so that even materials with a high melting point can be vaporized. Typically the focal point surface area is smaller than 1 cm.sup.2, so that only small vaporization zones are created. If therefore the electron beam is stationary or the speed with which it scans the surface to be vaporized is too low, the greater part of the electron beam energy goes into the depths of the material, which does not contribute to better vaporization.
The power distribution on the surface to be vaporized can be regulated with modern auxiliaries, whereby the layer thickness of the vapor-deposited material, for instance, can be optimized in a simple manner by changing the pattern of the beam scanning.
Layers applied by electron-beam vaporization are often less dense than comparable sputtered layers, and the properties of the layers can also be different. In order to improve the properties of the layers applied by means of electron-beam vaporization, additional plasma support can be added during the vapor-deposition process.
Due to the interaction of the electron beam with the residual gas particles, the pressure in a coating chamber and the spacing between the electron beam gun and the material to be vaporized, i.e., the beam length, must not exceed a prescribed value. For acceleration potentials of 20 to 50 kV, for instance, the pressure must not be greater than 10.sup.-2 mbar. The length of the electron beam should not exceed 1 m. If higher pressures or longer electron beam lengths are required, the acceleration potential should be increased.
A pressure increase at higher power levels can also be caused by the shield effect of material impurities, for instance, by H.sub.2 O or water of crystallization. Furthermore, some oxides break up in part into metal and oxygen. The pressure increase can change the layer properties or defocus the electron beam. The vaporization materials should therefore be optimized with regard to the shield effect of impurities and water.
Electron-beam guns with a power of up to 1000 kW and with acceleration potentials of up to 160 kV are available. For coating purposes, electron-beam guns with powers of 150 to 300 kW and acceleration potentials of 35 kV are generally employed. The electron-beam deflection and focusing are generally carried out by means of magnetic coils. Both the beam focusing and the beam deflection can be easily controlled by varying the currents flowing in the magnetic coil.
In general, scanning frequencies of more than 10 kHz are used in electron-beam welding. For coating applications, on the other hand, the customary frequency is around 100 to 1000 Hz, this frequency relating to the fundamental frequency. If harmonics are present, frequencies of, for instance, 10 kHz are included. Scanning frequency is understood to mean the frequency at which an electron beam moves back and forth between, for instance, two points on the surface of a crucible.
In the controlling of a high-powered electron beam, essentially the following aspects must be paid attention to: the power supply to the gun, the guidance of the electron beam inside the gun and guidance of the electron beam over the process surfaces.
Several methods of controlling a high-power electron beam are already known, in which there is provided a special deflection system (DE 42 08 484 A1) with sensors for detecting the point of incidence of the electron beam on a melt (EP 0 184 680, DE 39 02 274 C2, EP 0 368 037, DE 35 38 857 A1). Also, deflection systems with more than one electron beam (U.S. 4,988,844) or electron-beam positioning regulators with magnetic field sensors (DE 35 32 888 C2) have been proposed.
Also known is a control of a high-power electron beam carried out by means of a microprocessor, in which conventional hardware is operated by software that is designed for uniform beam dispersion and great flexibility in the carrying out of melting instructions or formulas (M. Blum, A. Choudhury, F. Hugo, F. Knell, H. Scholz, M. Bahr: Application of a New Fast EB-Gun Control System for Complex Melting Processes, EB Conference, Reno/USA, Oct. 11-13, 1995). The essential characteristics of the high-frequency controlled electron-beam system are a thermal camera and measuring unit for the element concentration in the gas phase. This control system can be applied in a variety of ways, for instance, for hearth melting of titanium or in drop melting of tantalum. It is also suited for simultaneous control of several melting furnace, which can be equipped with up to 5 electron-beam guns. With it, it is also possible to implement an electron-beam process with precisely defined surface temperature distribution even for asymmetric melting arrangements, for instance, in horizontal drop melting, where on one side, the material to be melted is supplied via a water-cooled copper trough, or in another electron-beam arrangement, where a high input energy results at one side due to the overflowing melt material. The control is also accomplished in this known arrangement by means of a conventional PC, which is operated by way of a software based on WINDOWS.RTM..
In a refinement of the above-described control of a high-power electron beam, an electron beam scanning and control system is used, with which the electron-beam scanning rate is directly controlled (M. Bahr, G. Hoffmann, R. Ludwig, G. Steiniger: New Scan and Control System (ESCOSYS.TM.) for High-Power Electron Beam Techniques, Fifth International Conference on Plasma, Surface Engineering, Garmisch-Partenkirchen, September 1996). This control system, which relies on so-called "internal intelligence," has two essential characteristics. One characteristic pertains to error compensation. Here the behavior of the electron beam is first "trained," wherein one starts on a screen with low power. After this "training process," the frequency attenuation and deflection errors of the electron-beam gun are automatically compensated for. A circular pattern of the beam remains a circle and not, say, an ellipse in the crucible, even at different angles of incidence. The size of this circle remains constant even if the scanning frequency is changed. The deflection error compensation is performed by applying a 2.times.n-dimensional polynomial function. The frequency attenuation is compensated with respect to amplitude and phase-angle rotation by application of the Fast Fourier Transform algorithm. Thus, not only geometric patterns, but even very precise patterns are compensated. Nonetheless, the system operates with a frequency limitation of 10 kHz, which permits cycle frequencies of up to 1 kHz. This minimizes the necessity of a frequency-attenuation compensation. Alongside the aforementioned error compensation, the direct input of the power compensation for a given surface is essential. With the known system, vapor-deposited layers of great uniformity can be achieved at a high speed. For a reactive Al.sub.2 O.sub.3 process, for instance, a coating speed of 10 m/sec is possible. Additional details on how the aforementioned deflection-error compensation and frequency-attenuation compensation are achieved were not given in the aforementioned presentation.
Starting from the above-described state of the art, it is an object of the present invention to make it possible to deflect the electron beam automatically and with error-compensation for preset scanning or power patterns.